A multi-slice X-ray CT apparatus is for obtaining projection data of an object by irradiating cone beams, i.e. pyramidal X-ray beams from an X-ray source to the object and measuring the X-rays which have passed through the object by an X-ray detector in which detection elements are disposed in 2-dimensional direction (channel direction and row direction).
Also, a single-slice X-ray CT apparatus is for obtaining projection data of the object by irradiating fan beams, i.e. fan-shaped X-ray beams from the X-ray source to the object by measuring the X-rays which have passed through the object by an X-ray detector in which detection elements are disposed in 1 array (in channel direction).
In either case, they reconstruct tomographic images of the object by rotating, around the object, the X-ray source and the X-ray detector which are disposed opposite to each other, acquiring projection data of the object from many directions, carrying out reconstruction filtering process for deblurring, and performing back projection on the projection data of the object.
Projection data is acquired in discrete positions of the X-ray source (hereinafter referred to as “view”), and the number of views for one rotation of the X-ray source are normally several hundred to several thousand. Also, projection data for one view is composed of the data of channel number X row number (row number for the single-slice X-ray CT apparatus=1) of the X-ray detector.
The projection data acquired in a predetermined view is referred to as “projection data in the relevant view”. Also, the operation for acquiring the projection data for the number of views necessary for reconstructing one piece of CT image is referred to as “scan”.
The X-ray CT apparatus is an image diagnostic apparatus which has high versatility capable of diagnosing in a wide range of an entire body, and its usability has further increased due to development of multi-slice function and speeding up of scanning function. However, because X-rays are used to be applied to the object, it is still necessary to make an effort to reduce radiation dose to the object as much as possible. In order to reduce the radiation dose to the object, it is important to accurately grasp and control the radiation dose at the time of the actual scanning.
The conventional X-ray CT apparatuses have configuration that CTDI (CT Dose Index) prescribed in IEC60601-2-44 is displayed as radiation dose upon scanning on a display device of a console prior to actual scanning.
However, CTDI value to be displayed here is set assuming an acrylic phantom having 160 mm of diameter in the head region and an acrylic phantom having 320 mm of diameter in the abdominal region.
For example, acrylic equivalent diameter in an abdominal region for an average adult is 250 mm, which causes about a 30% (=320/250-1) difference compared to CTDI. If the influence of the difference thereof on radiation dose is measured, for example, in the case of scanning with 120 kV of tube voltage, the patient suffers a loss of 25% due to unreasonable under-estimation.
In other words, the radiation dose displayed on the display device of the console in accordance with the IEC standard is merely a conversion of the radiation dose measured in advance using the reference phantom which is given one kind for each of head region and abdominal region considering the scan condition for actual application, which has a gap from the radiation dose considering the actual physique of the object.
As a technique for improving such error, the X-ray CT apparatus for calculating the distribution of X-ray absorption coefficient from the CT value distribution obtained by a CT apparatus to calculate actual radiation dose to the object is proposed in Patent Document 1. By such X-ray CT apparatus, it is possible to obtain radiation dose distribution in the object and estimate the radiation dose in the respective tissues.
Patent Document 1: JP-A-2005-074000